Tsunetaka Banba

Characterization of Fe-montmorillonite: A Simulant of Buffer Materials Accommodating Overpack Corrosion Product

Candidate materials as engineered barriers for high-level radioactive waste disposal include carbon steel for the overpack and bentonite for the buffer material. If these materials are selected, corrosion products from the overpack will migrate into the compacted bentonite. 1–3) Potential diffusing species of the corrosion products include Fe 2+ ions. The Fe2+ ions will be sorbed on montmorillonite, which is the major mineral component of bentonite. Some Fe 2+ ions may form more stable hydroxides and oxides. Radioactive nuclides leached from waste forms will migrate in such altered montmorillonite. An anticipated shortcoming of the alteration of montmorillonite is degradation of swelling property, which is liable to cause acceleration of nuclide migration. On the other hand, an expected advantage may be the immobilization of nuclides with high sorption affinities for iron compounds. However, the characteristics of such altered montmorillonite including retentivity for radioactive nuclides have yet to be clarified. Here we briefly report characterization of homoionic Fe 2+montmorillonite (Fe-montmorillonite) and its oxidized product that do retain Se(VI) for which natural montmorillonite such as Na +-type and Ca 2+-type has little retentivity. We used Fe-montmorillonite as a simulant of the buffer material in which Fe2+ ions diffused.

Formation and Evolution of Alteration Layers on Borosilicate and Basalt Glasses: Initial Stage

An experimentally altered (1–14 days, deionized water, 90°C), synthetic borosilicate glass and naturally altered (10 4 years, fresh water, 0–100°C), basalt glass were characterized by analytical electron microscopy to evaluate the development of alteration layers. Layers formed on borosilicate glass are initially amorphous. With increasing age, nontronite, chlorite, septechlorite, and/or stilpnomelane appear to precipitate and grow within the amorphous matrix and on the surface of the layer. Crystals are enriched in Fe, Co, and Ni, and depleted in Si relative to the surrounding amorphous matrix. Alteration layers on basalt glass are texturally and mineralogically similar to layers formed on borosilicate glass, but the degree of crystallinity is less.

Crystalline phases in a devitrified simulated high-level waste glass containing the elements of the platinum group

Abstract A borosilicate glass containing 20 wt% simulated high-level waste oxides was subjected to heat treatment at 700°C for 1000 h. Seven crystalline phases were newly formed by the treatment in addition to two phases, (Ru, Rh)O2 and (Pd, Rh, Te), which had already existed in the as-prepared simulated high-level waste glass. Among the new seven phases, five phases were certainly identified to be (RE)BSiO5, CeO2, SiO2, (RE)PO4 and (Sr, Ba, RE)MoO4. Of two unidentified phases, one was rich in silicon, chromium and rare earth elements (RE), and the other was rich in nickel and chromium. The crystalline phases of the elements of the platinum group facilitated the occurrence of the other phases and suppressed crystal growth.

Segregation of the elements of the platinum group in a simulated high-level waste glass

Segregation of the elements of the platinum group occurred during vitrification of the borosilicate glass containing 20 wt% simulated high-level waste oxides. The segregated materials were composed of two crystalline phases: one was the solid solution of ruthenium and rhodium dioxides and the other was that of palladium and rhodium metals also with tellurium. The segregated materials were not distributed homogeneously throughout the glass: (i) on the surface of the glass, there occurred palladium, rhodium and tellurium alloy alone; and (ii) at the inner part of the glass, the agglomerates of the two phases were concentrated in one part and dispersed in the other.

Development of ceramic waste forms for actinide-rich waste : Radiation stability of perovskite and phase and chemical stabilities of Zr- and Al-based ceramics

Abstract Ceramics are considered as most promising materials for conditioning of long-lived radionuclides because of their outstanding durability for long term. The Japan Atomic Energy Research Institute (JAERI) has developed ceramic waste forms, e.g. Synroc and zirconia-based ceramics, for the actinide-rich wastes arising from partitioning and transmutation processes. In the present study, α-decay damage effects on the density and leaching behavior of perovskite (one of three main minerals forming Synroc) were investigated by an accelerated experiment using the actinide doping technique. A decrease in density of Cm-doped perovskite reached 1.3 % at a dose of 9 × 10 17 α-decays·g −1 . The leach rates (MCC-1 leach test inpH ∼ 2 solution at 90°C for 2 months) of perovskite specimens with accumulated doses of 1.6 × 10 17 , 4.0 × 10 17 and 8.3 × 10 17 α-decays·g −1 were 1.7, 2.3 and 3.0 μ·m −2 ·day −1 , respectively. Application of zirconia- and alumina-based ceramics for incorporating actinides was also investigated by the experiments using non-radioactive elements (Ce and Nd) with an emphasis on crystallographic phase stability and chemical durability. The yttria-stabilized zirconia was stable crystallographically in the wide ranges of Ce and/ or Nd content and had excellent chemical durability.

Effects of Redox Condition on Waste Glass Corrosion in the Presence of Magnetite

Static corrosion tests of a powdered simulated waste glass were performed in the presence of magnetite under oxic and anoxic conditions, respectively. The corrosion tests under oxic conditions were performed in air, and those under anoxic conditions were performed in a glove box purged with mixed gas(Ar+5%H 2 ). The experimental results showed that the presence of magnetite can enhance glass corrosion under both oxic and anoxic conditions, and the enhancement under anoxic conditions was small compared to that under oxic conditions. A large portion of the Si and insoluble elements were sorbed onto the magnetite surface, and the sorption under anoxic conditions was small compared to that under oxic conditions. It was suggested that the enhancement of glass corrosion by magnetite results from the sorption or precipitation of silica on the magnetite surface, which can be greatly affected by redox condition. Under anoxic conditions, it was suggested that precipitation of amorphous silica on the magnetite surface may be the dominant process for enhancement of glass corrosion in addition to sorption.

Apparent diffusion coefficients and chemical species of neptunium (V) in compacted Na-montmorillonite.

Diffusion of neptunium (V) in compacted Na-montmorillonite was studied through the non-steady state diffusion method. In this study, two experimental attempts were carried out to understand the diffusion mechanism of neptunium. One was to establish the diffusion activation energy, which was then used to determine the diffusion process in the montmorillonite. The other was the measurement of the distribution of neptunium in the montmorillonite by a sequential batch extraction. The apparent diffusion coefficients of neptunium in the montmorillonite at a dry density of 1.0 Mg m-3 were from 3.7 x 10(-12) m2 s-1 at 288 K to 9.2 x 10(-12) m2 s-1 at 323 K. At a dry density of 1.6 Mg m-3, the apparent diffusion coefficients ranged between 1.5 x 10(-13) m2 s-1 at 288 K and 8.7 x 10(-13) m2 s-1 at 323 K. The activation energy for the diffusion of neptunium at a dry density of 1.0 Mg m-3 was 17.5 +/- 1.9 kJ mol-1. This value is similar to those reported for diffusion of other ions in free water, e.g., 18.4 and 17.4 kJ mol-1 for Na+ and Cl-, respectively. At a dry density of 1.6 Mg.m-3, the activation energy was 39.8 +/- 1.9 kJ mol-1. The change in the activation energy suggests that the diffusion process changes depending on the dry density of the compacted montmorillonite. A characteristic distribution profile was obtained by the sequential extraction procedure for neptunium diffused in compacted montmorillonite. The estimated fraction of neptunium in the pore water was between 3% and 11% at a dry density of 1.6 Mg m-3 and at a temperature of 313 K. The major fraction of the neptunium in the montmorillonite was identified as neptunyl ions sorbed on the outer surface of the montmorillonite. These findings suggested that the activation energy for diffusion and the distribution profile of the involved nuclides could become powerful parameters in understanding the diffusion mechanism.

Chemical Durability of Yttria-Stabilized Zirconia for Highly Concentrated TRU Wastes

Neptunium has an extremely long half-life and will be the main toxic element in the later stage of disposal. Yttria-Stabilized Zirconia (YSZ), with a fluorite structure, samples doped with Np-237 in high concentration (20, 30, 40 mol %) were fabricated (sintered in Air or Ar, at 1773 K, for 80 hours), and their leaching test (in deionized water, at 423 K, for 84 days) was carried out. The results indicated that the obtained leaching rates were much smaller than those of the Synroc and glass waste form, and that the increase in Np content did not cause any drastic changes in the leaching rates of Zr, Y, or Np. They were also compared to the results of YSZ doped with Ce or Nd used as surrogates for actinides. The work showed that YSZ doped with Np in high concentration has an excellent chemical durability

The leaching behavior of a glass waste form -- Part II: the leaching mechanisms

Soxhlet-type leaching experiments were carried out for 200 days and the leaching solutions analyzed by inductively coupled plasma spectroscopy and atomic absorption spectroscopy. The data of the solution analysis and the results of a previous study on the surface layers revealed the fact that elements in the waste glass were classified into three groups and were released into solution in accordance with the following mechanisms: Group I contained sodium, cesium, potassium, boron, and molybdenum; the release of the group I elements was controlled by diffusion and decomposition processes in the glass. Group II contained manganese, iron, nickel, zirconium, yttrium, lanthanum, cerium, neodymium, samarium, and dysprosium; the release of the group II elements was controlled by solubility of the sheet silicate formed in the surface layers. Group III contained silicon, aluminum, calcium, strontium, barium, magnesium, and chromium; the release of the group III elements was controlled by diffusion and decomposition processes in the glass, and was also affected by formation of the sheet silicate.

Corrosion Behavior of Simulated HLW Glass in the Presence of Magnesium Ion

Static leach tests were conducted for simulated HLW glass in MgCl2 solution for up to 92 days to investigate the dissolution mechanism of HLW glass under coastal repository condition. Under the condition that magnesium ion exists in leachate, the dissolution rate of the glass did not decrease with time during leaching, while the rate decreased when the magnesium ion depleted in the leachate. In addition, altered layer including magnesium and silica was observed at the surface of the glass after the leach tests. The present results imply that dissolution of the glass is accompanied with formation of magnesium silicate consuming silica, a glass network former. As a consequence, the glass dissolved with an initial high dissolution rate.